In the process of continuously melt-extruding thermoplastic material from an extrusion die, the extruded product exits the die at an elevated temperature and in a relatively soft state, unable to retain the shape imparted by the die. To assure that the final extruded product has the required exterior shape and dimensions, the exterior cross-sectional profile of the extruded product is cooled and calibrated by passing the extruded product through an extrusion calibrator. The extrusion system, thus, includes both the extrusion die and one or more extrusion calibrators. In each extrusion calibrator, the extruded product is cooled as it travels through a passageway generally similar in size and shape to that of the extrusion die. Each extrusion calibrator has ports and channels for the entry and circulation of a coolant medium (generally water) and vacuum ports for withdrawal of steam generated by cooling of the extruded product. As the extruded product travels through the extrusion calibrator passageway, a thin layer of water covers the extruded product outer surface. The water layer tends to cool the outer surface of the extruded product, producing a thin surface "skin" to further hold the product shape. The water layer also tends to lubricate the extruded product as it passes through the extrusion calibrator. The extrusion calibrator passageway may typically have a smaller cross-sectional size than the extrusion die passageway, further to shape the extruded product, which tends to contract as it cools. The extrusion calibrator passageway may also gradually reduce in cross-sectional size along the passageway, again to accommodate contraction of the extruded product on cooling. The amount of taper and the angle of taper will be determined based on the expected shrinkage of the specific plastic used for the extruded product.
Besides removing steam generated by the cooling extruded product, the vacuum holds the walls of the hollow or single wall extruded product in place against the interior walls of the passageway as the extruded product travels through the extrusion calibrator, further helping the cooling extruded product to retain its desired shape. The vaporization of water contacting the extruded product traveling through the extrusion calibrator removes a large amount of heat (BTUs) from the extruded product, and the steam is extracted through the vacuum exhaust ports. Such evaporation systems can remove heat from the extruded product at a much higher rate than possible with conventional heat transfer type extrusion calibrators. Because of the time required to cool the extruded product and the speed of extruded product travel required for economical production, a series of extrusion calibrators may be used to cool and shape the extruded product in an acceptable amount of time and at an acceptable throughput speed. Pullers positioned after the last extrusion calibrator control the extrusion speed by continuously drawing the formed extruded product exiting from the last extrusion calibrator. Examples of extrusion calibration systems are shown, for example, in U.S. Pat. Nos. 5,516,270, 5,514,325, 5,316,459, 5,288,218 and 4,468,369. Extruded products made by such processes typically have a uniform hollow cross-sectional shape along their longitudinal axis, including products such as automobile moldings, window parts, and pipe.
Automated EDM, Inc., the assignee of this application, has previously made available an extrusion calibrator, that was assembled from a series of thicker plates of a harder metal, such as steel, alternating with a series of thinner plates of a softer metal, such as aluminum. The thicker plates were each constructed with an aperture sized and shaped to a cross-sectional profile of an extruded product exiting from an extrusion die. The thinner plates were also each constructed with an aperture matched to that of the thicker plates. The thinner plates also were constructed with channels for introduction and circulation of coolant and vacuum. The thinner and thicker plates were assembled in alternation and retained by threaded rods inserted through the assembled plates. The apertures together defined the extrusion passageway and were shaped using wire electrical discharge machining (WEDM) to construct the passageway through the assembled plates. The vacuum and coolant channels were formed by using computer numerically controlled (CNC) milling technology. Holes for the threaded rods were drilled and reamed. With the plates assembled, the channels on the thinner plates provided pathways for coolant and vacuum circulation from sources exterior to the calibrator to the passageway to contact, cool and extract heat from the traveling extruded product. To provide an extrusion calibrator for an extruded product of a different cross-sectional profile requires the manufacture of an entirely new calibrator. Changing a calibrator of one passageway cross-sectional profile to one of a different profile requires disassembling and reassembling all vacuum and coolant access lines. If more than one extrusion calibrator of this earlier design is used in series, such as attached to a baseplate, all of the calibrators must be removed from the baseplate in order to change the passageway profile of the series of calibrators.